Abstract: Observational study and numerical simulation are conducted on the topography influences on the Mongolia cyclogenesis. The results show that the Mongolia cyclogenesis can be divided into two processes, namely triggering process and development process, according to the appearance of surface cyclonic cold front. In the development process, the baroclinicity is the main forcing mechanism causing the development of Mongolia cyclone. The topography influences focus on: 1) The Altai-Sayan complex mountains retarded the cold air in the lower troposphere so that the Mongolia cyclone developed slowly before the cold air completely toughed the lee side and after that it developed explosively. 2) The baroclinicity was enhanced by the complex mountains (steepened the isentropic surface), which made the enhancement of vorticity concentrated downward at the low troposphere and so reinforced the development of surface cyclone. 3) The complex mountains affected the position, range and strength of the lowel level jet. In the upper troposphere the isentropic potential vorticity (IPV) advection contributed to the cyclogenesis and its effect was limited.